1. Field of the Invention
The present invention relates to an organic electroluminescent (“electroluminescent” being hereinafter abbreviated to “EL”) element and a display panel using the organic EL element.
2. Background Art
EL elements utilizing electroluminescence are thin self-luminous elements which have excellent features such as high visibility and a wide view angle. Further, these luminescent elements are completely solid and thus also have excellent impact strength and other properties. Therefore, attention has been drawn to the utilization of this EL element as luminescent elements in various display devices.
EL elements are classified into inorganic EL elements using an inorganic compound as the luminescent material and organic EL elements using an organic compound as the luminescent material. Among them, organic EL elements can particularly significantly reduce applied voltage and can be easily reduced in size. Therefore, power consumption is small. Further, for organic EL elements, surface emission is possible, and luminescence of the three primary colors is also possible. By virtue of these advantages, for the organic EL elements, research and development have been made on the utilization of the organic EL elements as advanced luminescent elements (C. W. Tang and S. A. Van Slyk, Applied Physics Letters, vol. 51, P. 913 (1987)).
In the organic EL elements, a known construction is as follows. Specifically, the construction is basically anode/organic luminescent layer/cathode, and a hole injection transport layer or an electron injection layer is properly added to the above basic construction: for example, a construction of anode/hole injection transport layer/organic luminescent layer/cathode, or a construction of anode/hole injection transport layer/organic luminescent layer/electron injection layer/cathode. The term “charge transport” as used herein is used as meaning the highest concept of electron transport and hole transport. In these constructions, charge transport materials are important for the formation of a charge transport layer and a hole transport layer. The symbol “/” in the above terms means that the electrode or layer before “/” and the electrode or layer after “/” are stacked on top of each other.
The conventional organic EL element utilizes luminescence which is taken out from a fluorescent material. The organic EL element takes out energy, in an excited state produced by recombination of electron injected into the element with hole, as luminescence. The excited state produced as a result of a recombination of electron with hole is considered to be constituted by 25% of a singlet state and 75% of a triplet state.
In an organic EL element utilizing fluorescence, since only energy in a singlet state is used, in principle, the internal quantum efficiency is as low as 25%. In recent years, organic EL elements utilizing phosphorescence have attracted attention. In phosphorescent organic EL elements, not only energy in a singlet state but also energy in a triplet state can be utilized. Therefore, in principle, the internal quantum efficiency can be increased to 100%. In phosphorescent organic EL elements, phosphorescent luminescence can be taken out by doping a metal complex-type luminescent material containing a heavy metal such as platinum or iridium as a dopant which emits phosphorescence into a host material (M. A. Baldo et al., Nature, vol. 395, p. 151 (1998), M. A. Baldo et al., Applied Physics Letters, vol. 75, p. 4 (1999), and M. A, Baldo et al., Nature, vol. 403, p. 750 (2000)).
Luminescence of this phosphorescent dopant depends upon the host material. Basic properties required of the host material include that the host material has a hole transport capability and an electron transport capability, that the reduction potential of the host material is higher than that of the phosphorescent dopant, and that the level of energy in the triplet state of the host material is lower than the reduction potential of the dopant. In general, CBP (4,4′-bis(carbazol-9-yl)-biphenyl is suitably used as the host material (for example, Japanese Patent Laid-Open No. 68466/2003). The use of CBP in the organic EL element is known (for example, Japanese Patent Laid-Open No. 168443/1998).
In this organic EL element utilizing a phosphorescing material, however, a charge transport material such as CBP used as the host material is likely to be crystallized. Therefore, a large amount of doping (about 5 to 10% by mass) is necessary for preventing the crystallization. Further, film formation by coating is disadvantageously difficult. This has made it difficult to provide an organic EL element with a high luminescence efficiency using this CBP by the coating method.